1. Field of the Invention
This invention relates generally to an EUV radiation source and, more particularly, to an EUV radiation source that employs a target steering device to accurately steer the target droplets to the target vaporization area.
2. Discussion of the Related Art
Microelectronic integrated circuits are typically patterned on a substrate by a photolithography process, well known to those skilled in the art, where the circuit elements are defined by a light beam propagating through a mask. As the state of the art of the photolithography process and integrated circuit architecture becomes more developed, the circuit elements become smaller and more closely spaced together. As the circuit elements become smaller, it is necessary to employ photolithography light sources that generate light beams having shorter wavelengths and higher frequencies. In other words, the resolution of the photolithography process increases as the wavelength of the light source decreases to allow smaller integrated circuit elements to be defined. The current state of the art for photolithography light sources generate light in the extreme ultraviolet (EUV) or soft x-ray wavelengths (13-14 nm).
U.S. patent application Ser. No. 09/644,589, filed Aug. 23, 2000, entitled xe2x80x9cLiquid Sprays as a Target for a Laser-Plasma Extreme Ultraviolet Light Source,xe2x80x9d and assigned to the assignee of this application, discloses a laser-plasma, EUV radiation source for a photolithography system that employs a liquid as the target material, typically xenon, for generating the laser plasma. A xenon target material provides the desirable EUV wavelengths, and the resulting evaporated xenon gas is chemically inert and is easily pumped out by the source vacuum system. Other liquids and gases, such as krypton and argon, and combinations of liquids and gases, are also available for the laser target material to generate EUV radiation.
The EUV radiation source employs a source nozzle that generates a stream of target droplets. The droplet stream is created by forcing a liquid target material through an orifice (50-100 microns diameter), and perturbing the flow by voltage pulses from an excitation source, such as a piezoelectric transducer, attached to a nozzle delivery tube. Typically, the droplets are produced at a high rate (10-100 kHz) at the Rayleigh instability break-up frequency of a continuous flow stream. The droplets may be emitted from the nozzle into a vacuum, where rapid evaporation and freezing of the droplets will result, or they may be ejected into a buffer gas at an appropriate pressure and temperature to control the rate of evaporation of the droplets.
To meet the EUV power and dose control requirements for next generation commercial semiconductors manufactured using EUV photolithography, the laser beam source must be pulsed at a high rate, typically 5-10 kHz. It therefore becomes necessary to supply high-density droplet targets having a quick recovery of the droplet stream between laser pulses, such that all laser pulses interact with target droplets under optimum conditions. This requires a droplet generator which produces droplets within 100 microseconds of each laser pulse.
Droplet generators, including downstream differentially pumped cavities, are relatively massive and employ many connections for coolant, vacuum and electrical lines. Thus, weight and configuration constraints make the droplet generator difficult to position, and consequently severely limits its positioning response time. Further, the orientation of the droplet generator relative to the target location may be required to be off axis.
In accordance with the teachings of the present invention, an EUV radiation source is disclosed that employs a steering device for steering a droplet stream generated by a droplet generator to a target area. The droplet generator directs the stream of droplets in a certain direction that is sensed by a position sensor. The sensed position of the droplet stream is sent to an actuator that controls the orientation of the steering device. The droplet stream impinges the steering plate and is deflected therefrom towards the target area.
Additional objects, advantages and features of the present invention will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings.